[ATM] Cu3Sn Progress

[atmer at flash.net (Anthony Stillman)]

I've sanded the 64 mm speculum metal blank into shape.  Hogging with 60
grit sandpaper, I worked my way up to 1500 grit.  The last few sanding
steps I did at the Chabot Mirror Making Workshop.  To do a quick test, I
polished the mirror with a paper towel, my finger and cerium. Instructive,
but disappointing.  The light focused and the figure was, well it was.

To actually polish the mirror, I made a pitch lap with recycled 73,
softened with a little bee's wax.  I polished with cerium, and immediately
ran into a problem.  A film of copper colored metal built up on the lap.
After this happened the feel of the polishing action changed.  Like there
was grit in the slurry.  Examining the lap, I noticed scratches through the
metal film and pitch.  I kept polishing.  Soon there were scratches on the
mirror too.  I scraped the lap, warm pressed it, and started in again, but
with rouge.  It took longer for the metal film to develop, but it did.
After four more scrapes and presses I started looking for another solution.
Eventually I tried rubbing the lap with bar soap, and presto change-o, the
scratch problem went away.  The metal film still builds up on the lap, but
there is less of it. I have to re-soap ever ten minutes or so.

It's taken quite a while to polish this little mirror and it's not done
yet.  Probably in part this is due to my using rouge. I've yet to try
cerium on a soaped lap.  I'll court that failure with the next mirror.

As a measure of what constitutes polished out, I've tracked the
obliteration of one nasty scratch.  I think another hour (read 5) will do
it.

There are numerous defects in the mirror face.  The most obvious are the
pits (porosity).  They are a result of my casting method.  I used a solid
brass mold, its interior coated with yellow ocher.  The casting porosity is
a consequence of gas trapped between the mold and liquid metal being forced
into the metal before it solidified.

I've alloyed up another 1.4 kg of metal.  Soon I'll break up the billets,
melt the metal in a  furnace and pour a 100 mm diameter blank.

I have about 2 kg of tin left.  Enough for 6 kg of metal. If I'm successful
in refining my methods, I'll try to make a single 175 mm diameter blank
from this.

On my return form travels (again I was able to transport a telescope with
out any TSA agents pee-ing their pants) I find that my good friend Richard
Schwartz has changed the balance of how much tin I have to use.  Also, with
the 64 mm mirror, I was able to project an image of a Sun spot today.

Anthony